The goal of this project is to identify and characterize B lymphocyte subpopulations and membrane molecules. Monoclonal antibody F1-1O detects a determinant (termed Ly-39.1) preferentially expressed on activated B lymphocytes. Expression peaks 60 hours after activation using LPS. The Ly-39.1 determinant is also expressed on splenic B lymphocytes and a subpopulation of bone marrow cells but at much lower levels and is absent from thymocytes, splenic T lymphocytes, and activated T lymphocytes. Mapping studies using BXD recombinant inbred mice indicate a gene controlling expression of the Ly-39.1 determinant is located on chromosome 17 between hbap-4 and acry-1. These results together with a unique strain distribution pattern suggest that Ly-39.1 is detecting a previously undescribed determinant selectively expressed on activated B lymphocytes which may be a receptor for lymphokines. A new hybridoma screening method has been developed using particle concentration fluorescence which is capable of detecting antibodies binding to molecules which are expressed at very low levels (100-300 molecules per cell). Studies of B lymphocyte subpopulations indicate that large "activated" B lympho- cytes obtained directly from mice or B lymphoblasts induced in vitro with F(ab')2 anti-mu significantly augment the responses of small "resting" B lymphocytes to F(ab')2 anti-mu and lymphokines. Proliferation was augmented 2-4 fold while antibody production was augmented 4-5 fold. This effect was specific for "activated" B lymphocytes in that other cell types did not have this effect. Kinetic experiments revealed that the augmenting signal was effective after stimulation via antigen receptors but prior to the effects of lymphokines. The augmenting effect does not appear to be genetically restricted. Investigation of the nature of the signal revealed that neither supernatants nor plasma membranes from activated B cells alone augmented responses but both together did. These studies suggest that interactions between B lymphocytes are important in regulating humoral immune responses.